In fabricating hybrid integrated circuits and optical subassemblies, it is necessary to achieve high-strength bonds having high thermal and electrical conductivity between submount metallization parts and parts bonded to the submount. One method commonly used to achieve such bonds is soldering. For example, a metal mixture (a solder), is disposed between a laser chip and a substrate, and the solder is melted in contact with the two parts and then resolidified. Generally, resolidification is achieved via thermodynamic bonds, that is, by reducing the temperature of the parts and the solder, which causes the material of the solder to freeze with the material of the parts. If the parts are never again exposed to a temperature high enough to melt the solder, the bond remains solid.
One disadvantage of soldering methods involving thermodynamic bonds is that they cannot be used with a high probability of success if after the initial bond is formed, the parts must be raised to a temperature close to or above the softening point of the solder. For example, with a solder comprised of gold and tin, the solidus line of the composition may be at approximately 280 degrees Centigrade. With thermodynamic bonds, the integrity of the solder may be jeopardized if the parts are later raised above that solidus line. This is an especially difficult problem when the parts must be maintained in a precise dimensional alignment, which is the case when lasers are bonded in an optical system. Thus, efforts recently have been directed toward developing bonds that will remain intact even at temperatures above the melting point of the original solder composition. In S. Bader, W. Gust, and H. Hieber, "Rapid Formation of Intermetallic Compounds by Interdiffusion in the Cu-Sn and Ni-Sn Systems," ACTA METAL. MATER. Vol. 43 (1995), at 329-337, there is discussed the use of binary intermetallic formulations that reportedly provide a solder system in which the usage temperature may be greater than the melting temperature of the original solder materials. Bader et als. teaches the use of a binary formation with no controlling elements.
Kinetic bonds (bonds controlled with the use of chemical reaction rates and controlling elements), may prove advantageous over thermodynamic bonds or pure binary bonds. With kinetically-controlled bonds, the timing of chemical reactions (kinetics) controls the degree of melting, oxidation, or chemical transformation and thereby the wetting, flow, and strength of the solder bond. For example, a solder comprising one or more layers of elements may be incorporated on one part or the other. Additional layers of solder elements may be added to the solder to cause the melting point of the solder to vary as the elements of the parts are absorbed within the solder. These additional layers are referred to herein as "quenching layers." Thus, freezing may be induced with the incorporation of quenching layers of additional elements rather than by solely lowering the temperature. In such solder arrangements, the bond can sometimes be made so that it will remain solid even at temperatures above the melting point of the original solder composition.
For example, a kinetically-controlled bond involving the use of quenching layers known in the art involves the use of a gold-tin (AuSn) solder in the bonding of gold (Au) or gold-covered parts. In the bonding of two parts, such as a laser chip and an optical sub-assembly substrate, a mixture of gold and tin is commonly used for a solder layer which is applied to one part, e.g., the substrate. In thin film applications, the solder layer typically will comprise a plurality of layers, such as alternating layers of gold and tin. The solder layer is melted by applying a temperature to it and the substrate that is slightly above the eutectic point of the solder, to ensure complete melting of the solder layer. Once the solder layer is melted, a quenching layer (such as an additional layer of gold), is added, and the second part (the laser chip), may then be applied on the quenching layer. Alternatively, one or more additional solder layers or quenching layers may be added before the second part is applied. As the solder and quenching layers freeze, they bond the two parts together.
With this process, the quenching layer increases the bond strength and provides some added control over the bonding. As the quenching layer is incorporated into the solder layer and the bond thus formed, the temperature at which the mixture is solid (the solidus line) rises as a function of the atomic fraction of gold in the mixture. Hence, the incorporation of additional gold (the quenching layer), which is not part of the original solder layer allows one to adjust the rate at which the material is brought to freezing, as opposed to by simply lowering the temperature. However, this type of solder bond remains difficult to control under most circumstances, because the additional elements often will be rapidly incorporated into the solder layer, such that the bond must be made quickly to avoid premature freezing. There generally remains a need for obtaining better control over this soldering process.
Another drawback with the above-described method (besides the rapidity of the reaction and the need to obtain greater control over the process), involves oxidation of the solder materials. Oxidation may occur before the bond is formed or during storage. Also, oxide layers and intermetallic compounds formed on the surface of the solder layer prior to bonding can cause a lack of, or a deficiency in, the metal-metal contact and lead to the formation of a bond with undesirable characteristics such as poor strength or low electrical conductivity. Oxidation is particularly problematic when tin (Sn) is used in the solder layer. When tin is used, it may diffuse to the surface of the solder and cause the formation of a layer of SnO.sub.2. This may cause wetting problems during bonding, as the oxide (SnO.sub.2) is moisture sensitive. The diffusion of tin in gold and the mutual diffusion of tin and gold is well-known to be rapid even at room temperature. See. e.g., L. Buene, "Interdiffusion and Phase Formation at Room Temperature in Evaporated Gold-Tin Films," THIN SOLID FILMS 47:159 (1977); S. Nakahara, R. McCoy, L. Buene, and J. M. Vandenberg, "Room Temperature Interdiffusion of Au/Sn Thin Film Couples," THIN SOLID FILMS 84:185 (1981); V. Simic and Z. Marinkovic, "Thin Film Interdiffusion of Au and Sn at Room Temperature," JOURNAL OF THE LESS COMMON METALS 51:177 (1977). Even if the solder layer is pure gold, such oxidation and wetting problems may arise.
Various methods are commonly used to prevent or reduce oxides, for example, storage in inert ambients such as argon; storage at very cold temperatures; the use of fluxes; or reducing the ambient atmospheres during the bonding process. These methods generally are rather costly due to storage or bonding conditions or the equipment required, and they may potentially impact on reliability or process safety. Some recent studies have explored the incorporation of nickel into the soldering structure as a barrier layer to seek to address difficulties encountered with interdiffusion. C. H. Lee, Y. M. Wong (an inventor herein), C. Doherty, K. L. Tai, E. Lane, D. D. Bacon, F. Baiocchi, and A. Katz, "Study Of Ni As A Barrier Metal In AuSn Soldering Application For Laser Chip/Submount Assembly," J. APPL. PHYS. 72:3808 (1992).
For further background regarding solder bonding, see U.S. Pat. No. 5,559,817, entitled "Compliant Layer Metallization," issued Sep. 24, 1996, to G. Derkits, an inventor herein, and J. Lourenco and R. Varma, which patent was assigned to Lucent Technologies, Inc., the assignee herein, and is incorporated herein by reference.
The present invention is addressed to improved methods of forming solder bonds and improved solder bond compositions. With the method of the present invention, greater control is achieved over the reaction rates and bonding process so improved bonds may be formed remaining solid even at temperatures above the melting point of the original solder composition. Oxidation of solder materials is also controlled. Further advantages may appear more fully upon consideration of the illustrative embodiments described in detail below in connection with the accompanying drawings.